In one known method of removing charges from an electrostatically charged workpiece, the charges are removed by releasing positive and negative ions from an ionizer toward the workpiece fed into a charge removing area, and neutralizing the positive or negative charges carried on the workpiece by the ions having the polarity opposite to that of the charges on the workpiece.
The ionizer generally has a positive electrode needle and a negative electrode needle. By applying a positive pulse-like high voltage to the positive electrode needle and applying a negative pulse-like high voltage to the negative electrode needle, corona discharge is produced to generate positive and negative ions from both the electrodes.
When charges are removed from a workpiece by using such an ionizer, efficiency of a charge removal process can be increased by releasing, corresponding to the polarity of the charges on the workpiece, a larger amount of ions which have the opposite polarity. Depending on the condition of the charge removal process, however, it is impossible to properly confirm in which one of positive and negative polarities is charged the workpiece. In view of that situation, therefore, the charge removal process is desirably adaptable for any case regardless of whether the fed workpiece is positively or negatively charged. One conceivable means for satisfying such a demand is to make pre-adjustment of positive and negative ions released from the ionizer so that an ion balanced state, i.e., a state of the positive and negative ions being substantially equal to each other in number, is obtained in advance. In that case, the pre-adjustment is required to be reliably carried out by simple means.
On the other hand, as one example of a method of adjusting an ion balance, Patent Document 1 discloses a technique of adjusting the ion balance by detecting a current, which flows through a ground line depending on the amount of positive and negative ions consumed when charges are removed from a workpiece, by a current sensor, and controlling positive and negative high-voltage generation circuits such that ions with the required polarity are generated in larger amount.
Also, Patent Document 2 discloses a technique of taking an ion balance by arranging a current detecting electrode between positive and negative electrode needles, detecting an ion current, which flows between both the electrode needles when charges are removed from a workpiece, by the current detecting electrode, and adjusting a voltage or pulse width, which is applied to the electrode needles, depending on the polarity of the ion current and the difference in ion amount.
However, because those disclosed techniques are each intended to take an ion balance by detecting the current flowing through the ground line or between both the electrode needles, any of those techniques cannot directly confirm whether positive and negative ions are actually well balanced. Further, if the current is changed due to some other factor than ions, there is a risk that a malfunction may occur and the ion balance may be lost in reverse. Thus, those techniques have a problem in reliability.
Further, Patent Document 3 discloses a technique of employing an electrostatic potential sensor for measuring an electrostatic potential of a charge removal target (workpiece) and an electrostatic potential sensor for measuring an ion balance around an ionizer, and adjusting the amount of ions released from the ionizer by using those two electrostatic potential sensors based on measured values of both the sensors during a process of removing charges from the workpiece. More specifically, during the first period of the charge removal process in which the charge potential of the workpiece is sufficiently high, ions with the polarity opposite to that of the charges are irradiated to quickly remove the charges from the workpiece. During the final period of the charge removal process in which the electrostatic potential of the workpiece is low, ions in the ion balanced state are irradiated to remove the charges from the workpiece.
According to the technique described above, however, because the polarity of the charges on the workpiece and the ion balance around the ionizer are measured by using the two electrostatic sensors and the amount of irradiated ions is controlled depending on the polarity of the charges on the workpiece, the construction and control of an apparatus are complicated. In addition, because the ion balance is measured during the process of removing charges from the workpiece, i.e., in the presence of the charged workpiece, and the amount of irradiated ions is controlled based on the measured result, an influence of the charged workpiece is caused as a disturbance, thus resulting in a difficulty in actually attaining the proper ion balance. Particularly, when charged workpieces are fed successively with short intervals, the adjustment of the ion balance is not finished in time and the charge removal process becomes hard to perform with reliability.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-135293
Patent Document 2: Japanese Unexamined Patent Application Publication No. 3-266398
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-217892